An Anaplerotic Role for Mitochondrial Carbonic Anhydrase in Chlamydomonas reinhardtii  

Abstract

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO 2 concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO 2 concentrations decreases when the external NH 4 ϩ concentration decreases, to the point of being undetectable when NH 4 ϩ supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO 2 by increasing the NH 4 ϩ concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO 3 Ϫ for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.In algae and plants, the tricarboxylic acid (TCA) cycle plays a fundamental biosynthetic role (Beardall and Raven, 1990). The removal of intermediates from this cycle to feed other biosynthetic pathways (of which amino acid synthesis is often quantitatively the most important) requires that the cycle is replenished of its intermediates via anaplerotic reactions (Beardall and Raven, 1990; Norici and Giordano, 2002). The anaplerotic reactions make use of inorganic carbon to build the C4 compounds in demand in the TCA cycle, via ␤-carboxylation (Beardall and Raven, 1990; Norici and Giordano, 2002). The provision of inorganic carbon for these reactions can be crucial to sustain amino acid and protein synthesis (among others; Norici and Giordano, 2002). Anaplerotic ␤-carboxylation therefore represents a pivotal intersection among the metabolisms of C and N. Consequently, mechanisms must exist to ensure that there is sufficient inorganic carbon to maintain anaplerosis at an appropriate rate, especially in conditions in which the dissolved inorganic carbon (DIC) in the cytosol may be limited, competition with other DIC-requiring pathways (mostly photosynthesis) is significant, and N assimilation is fast. Respiration and photorespiration are conveniently located (spatially and functionally) sources of CO 2 to supply reactions that replenish the TCA cycle. A mechanism that recovers respiratory CO 2 would therefore be a very effective way to ensure an appropriate flux of C to the TCA cycle via anaplerosis. However, respiration and photorespiration produce DIC in the form of CO 2 , whereas many ␤-carboxylases such as phosphoenolpyruvate carboxylase (PEPc) and pyruvate carboxylase (PC) require HCO 3 Ϫ (Chollet et al., 1996; Norici and Giordano, 2002). Thus, if the uncatalyzed rate of CO 2 conversion to HCO 3 Ϫ is not sufficiently high, enzymatic hydration of CO 2 to HCO 3 Ϫ may be necessary (Raven and Newman, 1994; Huang and Chapman, 2002). Reactions of this sort are catalyzed by carbonic anhydrases (CAs), whose activity is wi...

Interactions between CCM and N2 fixation in Trichodesmium

“…In Chlamydomonas reinhardtii, for instance, the CCM was down-regulated under N depletion (Giordano et al 2003), which may reflect lower carbon demand and serves to maintain the C:N ratio relatively constant. Controlling elemental stoichiometry is of fundamental importance for ensuring optimal functioning of the enzymatic machinery (Beardall and Giordano 2002).…”

Algal evolution in relation to atmospheric CO 2 : carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

“…All other well-investigated CCMs seem to involve 'normal' carbonic anhydrases, i.e. those catalysing the equilibration of CO 2 and HCO À 3 [11,67,111]: this is the case for 'active CO 2 influx' in cyanobacteria, which involves a carbonic anhydrase in the carboxysome as well as the energized conversion of CO 2 to HCO À 3 at the thylakoid membrane, which is effectively a unidirectional carbonic anhydrase [56].…”

A Metabolomic Approach to Study Major Metabolite Changes during Acclimation to Limiting CO2 in Chlamydomonas reinhardtii

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